1. Field of the Invention
The present invention relates generally to a skewness detector for detecting the skewness of an object with respect to a predetermined orientation. More specifically, the invention relates to a reflection-type skewness detector for use in optical disk players, optical video disk players, compact disk players and the like. More particularly, the invention relates to a skewness detector compact enough to be installed in an optical block of an optical disk player or the like.
2. Description of the Prior Art
Generally, in an optical disk reproducing apparatus, a laser beam is focussed onto the recording surface of an optical disk by an objective lens so as to reproduce a signal recorded thereon. In this case, resolution is determined by how much area the laser beam covers, or the diameter of the beam spot. Accordingly, the maximum value of the diameter of the beam spot is designed to be less than a specified value. The diameter of the beam spot is determined by the number of wavelengths the laser light travels from its source and the ratio between the focal length of the objective lens and its diameter. The ratio between the focal length of an objective lens and the diameter of the lens is generally represented by a "numerical aperture (NA) value". Conventional helium-neon lasers used as the light source in optical disk players, make the optical disk player bulky and relatively expensive. Accordingly, it is a recent tendency to use semiconductor lasers as the light source because they are more inexpensive than helium-neon lasers and thus help make the apparatus more compact. However, semiconductor lasers exhibit a wavelength of 780 nm which is longer than that of the helium-neon laser, i.e. 623.8 nm. For this reason, in order to limit the diameter of beam spot of the semiconductor laser so as to provide the same resolution as that of a helium-neon laser, the NA value of the objective lens must be increased to, for example, about 0.5.
However, when the NA value of the objective lens is increased as set forth above, if the optical axis of the laser beam is not precisely perpendicular to the recording surface of the optical disk, crosstalk from adjacent tracks on the disk can become a serious problem.
In order to eliminate or significantly reduce crosstalk in reproduction of the optical disk, there has been proposed a servo mechanism for an optical head block for adjusting the optical axis to precise perpendicularity with the recording surface of the optical disk. This servo mechanism employs a skew detector for detecting the relative skew between the recording surface of the optical disk and the optical head.
Recently, various head-block-mounted skew detectors have been proposed for monitoring relative skew between the disk and the optical head block. For example, in one typical skew detector, a diffused light source comprises a lamp and a diffusion plate. The diffusion plate is formed as a window of quadrilateral shape and the periphery of the window is masked by a light shielding portion. A light receiving device receives the light emitted by the diffusion light source and reflected by the disk. The light receiving device comprises a cylindrical member, a multi-element photo detector disposed in the floor of the cylindrical member, and a condenser lens supported by the cylindrical member between the photo detector and the recording surface of the disk. The diffused light source and the light receiving device are aligned along an axis perpendicular to the desired plane of the disk. Since the radial skewness of the disk is to be detected, the diffused light source and the light receiving device are aligned perpendicular to the radii of the disk. The diffused light source and the light receiving device are arranged such that the light receiving device receives an asymmetrical reflection from the recording surface of the disk if the disk is at all oblique or skewed.
This type of skew detector is too bulky to be installed in optical head blocks. This space problem may be resolved by employing the skew detector disclosed in the Japanese Patent First Publication (Tokkai) Showa (Appln. 58-140139), in which the diffused light source and the light receiving device are housed within a single cylindrical housing. The housing also supports a lens. The diffused light source and the lens are so arranged as to focus the light reflected by the recording surface of the optical disk onto the light receiving device. The light receiving device comprises a pair of light sensitive components so that light image can be formed on the pair of light sensitive components of the light receiving device. If the optical disk is skewed or deformed, the image formed on the light sensitive components will be shifted so that part of the image will not fall on the light sensitive components. The light sensitive components are adapted to produce an electric signal proportional to the amount of the light received. Therefore, when the disk is skewed or deformed, the signal values of the pair of light sensitive component will differ. This difference represents the skewness of the disk.
However, in this arrangement, since the diffused light source and the light receiving device are offset from the optical axis of the lens, the distance between the lens and the optical disk influences its detection characteristics. This, in turn, means that the components of the detector must all be fabricated with low tolerances and that assembly must also be highly accurate.